1. Field of the Invention
The present invention relates to an information recording/reproducing device with a high recording density.
2. Description of the Related Art
In recent years, compact portable devices have been widely used worldwide and, at the same time, a demand for a small-sized and large-capacity nonvolatile memory has been expanding rapidly along with the extensive progress of a high-speed information transmission network. Among them, particularly a NAND type flash memory and a small-sized HDD (hard disk drive) have rapidly evolved in recording density, and accordingly, they now form a large market.
On the contrary, some ideas for a new memory have been proposed, with the goal of greatly increasing the limit of recording density.
For instance, PRAM (phase change memory) adopts a principle in which materials capable of taking two conditions, an amorphous condition (OFF) and crystalline condition (ON), are used as recording materials, and these two conditions are caused to correspond to binary data “0” and “1” to record data.
Write/erase is performed in such a way that, for instance, the amorphous condition is prepared by applying a large power pulse to the recording material, and the crystalline condition is prepared by applying a small power pulse to the recording material.
A read is performed by causing a small read current to flow in the recording material to the degree that the write/erase is not generated, followed by measuring an electric resistance of the recording material. The resistance value of the recording material in the amorphous condition is larger than the resistance value of the recording material in the crystalline condition, and its ratio is in the degree of 103.
The greatest feature of the PRAM lies in a point that, even though element size is reduced to about 10 nm, the element can be operated. In this case, since the recording density of about 10 Tbpsi (tera bytes per square inch) can be realized, and accordingly, this is one of candidates for realizing increased recording density (for instance, refer to JP-A 2005-252068 (KOKAI)).
Further, a new memory has been reported which is different from the PRAM but has a very similar operation principle to the PRAM (for instance, refer to JP-A 2004-234707 (KOKAI)).
According to this report, a representative example of a recording material to record data is nickel oxide, in which, like the PRAM, the large power pulse and the small power pulse are used for performing the write/erase. There has been reported an advantage that the power consumption at the time of the write/erase becomes small as compared with the PRAM.
Until now, although the details of an operation mechanism of the new memory are not clear, reproducibility is confirmed, and thus this is noticed as one of the candidates for the increased recording density. Further, some research groups are attempting to clarify the operation mechanism.
In addition thereto, proposed is a MEMS memory using MEMS (micro electro mechanical system) technology (for instance, refer to P. Vettiger, G. Cross, M. Despont, U. Drechsler, U. Durig, B. Gotsmann, W. Haberle, M. A. Lants, H. E. Rothuizen, R. Stutz and G. K. Binng, IEEE Trans. Nanotechnology 1, 39 (2002)).
In particular, the MEMS memory, called Millipede, has a structure in which array shaped cantilevers face a recording medium to which an organic substance is applied, and a probe at a tip of the cantilever comes into contact with the recording medium with appropriate pressure.
A write is performed by selectively controlling the temperature of a heater added to the probe. That is, when increasing the temperature of the heater, the recording medium is softened, and then, depressions are formed on the recording medium because the probe forms dents in the recording medium.
A read is performed by scanning the probe on a surface of the recording medium while causing a current to flow through the probe to the degree that the recording medium is not softened. When the probe sinks into the depression of the recording medium, temperature of the probe decreases, and the resistance value of the heater increases, so that it is possible to sense the data by reading this change of resistance value.
The greatest feature of the MEMS memory such as the Millipede lies in a point that since it is not necessary for each recording part to provide wiring to record bit data, the recording density can be improved remarkably. Under existing circumstances, a recording density of about 1 Tbps has already been achieved (for instance, refer to P. Vettiger, T. Albrecht, M. Despond, U. Drechsler, U. Durig, B. Gotsmann, D. Jubin, W. Haberle, M. A. Lants, H. E. Rothuizen, R. Stutz, D. Wiesmann and G. K. Binng, P. Bachtold, G. Cherubini, C. Hagleitner, T. Loeliger, A. Pantazi, H. Pozidis and E. Eleftheriou, in Technical Digest, IEDM03 pp. 763 to 766).
Further, subsequent to the Millipede, recently, performed are attempts to achieve a large improvement concerning power consumption, recording density or working speed while combining the MEMS technique with a new recording principle.
For instance, proposed is a system in which a ferroelectric layer is provided on the recording medium, and recording of the data is performed by causing dielectric polarization in the ferroelectric layer by applying a voltage to the recording medium. Theoretically, this system is predicted to be able to utilize one crystal as a unit (recording minimum unit) for recording one byte of data.
If the recording minimum unit is equivalent to one unit cell of the crystal of the ferroelectric layer, the recording density rises to a phenomenal approx. 4 Pbpsi (peta bytes per square inch).
The MEMS memory which stores data by using the ferroelectric layer is operated by a well-known principle, but the MEMS memory is not realized at the present time.
One of the biggest reasons is that the electric field from the recording medium is shielded by ions in an air. In other words, reading does not execute, because the electric field from the recording medium is not sensed.
The other reason is that a lattice defect in a crystal shields charges, because charges caused by the lattice defect are transferred to the recording part.
About the problem of the electric shield by ions in an air, recently, based on development of a read system using SNDM (scanning nonlinear dielectric microscope), the new memory has advanced considerably toward practical use (for instance, refer to A. Onoue, S. Hashimoto, Y. Chu, Mat. Sci. Eng. B120, 130 (2005)).